The Science and Technology of Materials in Automotive Engines Part 6 pps

Increasing the tension of the valve springwill increase reactive force, helping to prevent irregular motion of the valves.However, the high reactive force will result in high contact pre

The oval shape of the cam lobe determines the lift (displacement) of inletand exhaust valves The valve itself has an inertial mass If the curved shape

of the cam lobe surface is not designed appropriately, then the valve cannotaccurately follow the contour and this will result in irregular motion This islikely to occur at high revolutions Lighter moving parts in the valve trainwill enable high-speed revolutions Increasing the tension of the valve springwill increase reactive force, helping to prevent irregular motion of the valves.However, the high reactive force will result in high contact pressure on thecam lobe, so the cam lobe should have high wear resistance

It is essential that adequate amounts of lubricating oil are supplied to thecam lobe The contact between the curved surface of the cam lobe and theflat face of the valve lifter (bucket tappet) generates high stress,1 and thereforeboth parts require high wear resistance where contact occurs In the DOHCmechanism, the cam lobe makes contact with the head of the valve lifterdirectly or via a thin round plate (pad or shim), which is positioned on thevalve lifter head The high contact pressure means a much harder material isneeded for the shims.The SOHC mechanism uses rocker arms (Fig 5.3) Theface that is in contact with the cam lobe also needs to have good wear resistance

The reactive force of the valve spring must be set high in order to maintainsmooth motion and generate high revolutions, as discussed above Themaximum permissible surface pressure, usually regarded as the decisiveparameter limiting cam lobe radius and the rate of flank-opening, currentlylies between 600 and 750 MPa, depending on the materials used.2

When the camshaft is operating at high revolutions, contact pressure isreduced by the inertia of the valve lifter Under these conditions, the oil film

on the running face is maintained most easily, providing hydrodynamiclubrication Contact pressure is therefore highest and lubrication mostchallenging when the engine is idling Figure 5.63 summarizes the basicrelationships between the factors that influence the tribology of the camshaft

50 mm

5.4 Camshaft installing a drive sprocket at the center The cam lobe converts rotation into reciprocating motion.

Generating accurate valve motion Operating at high rotational velocity Precise shape with less cost

Required functions for materials

High dimensional accuracy

High rigidity to prevent abnormal torsion & bending W

cam lobe under high contact pressure Durability of shaft High rigidity for torsion & bending Lightweight High shapability

High machinability High Y

Lubricating oil condition

Thermal effect (including friction heat) 1.

Clearance change caused by thermal expansion

Lubrication function 1 Oil quantity 2 Oil property 3 Oil temperature Load on the sliding surface 1.

Unbalanced load caused by fluctuation of loading position

Dimension of rubbing surface 1.

Dimensional accuracy of machining 1.

The PV value is a product of the pressure (P) and relative slip speed (V) at

the running surface, evaluating the severity of lubrication and friction conditions The higher the value, the more severe the working conditions.

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and valve lifter, and which can therefore cause problems that result in wear

at the point of contact

Figure 5.7 shows an example of flaking at the head of a DOHC valvelifter Flaking is caused by surface fatigue The Hertzian stress reaches itshighest value just under the contact surface, frequently resulting in fatiguecracks that then cause flaking (see also Chapter 9) In Fig 5.7, the surfacehas peeled off to reveal the cavities underneath, a typical failure under highcontact pressure

5.7 Flaking appearing in the valve lifter head Flaking is a type of wear where the face comes off like a flaky powder.

Pitting is another surface fatigue phenomenon Pitting normally manifestsitself as small holes and usually appears under high contact pressures Figure5.83 summarizes the main reasons why pitting occurs in the cam lobe and thefactors that affect each of these reasons The increased temperature at therunning surface that results from increased friction lowers the viscosity ofthe lubricating oil, making it less efficient Under these conditions, the matingmetal surfaces lose their protective oil film and come into direct contact.Wear can appear on either the tappet or the cam lobe It is very important tochoose an appropriate combination of materials

The function of the shaft itself is also very important The torque from thecrankshaft drives the camshaft, so the shaft portion is under high torque andtherefore must have high torsional rigidity Figure 5.9 shows a section taken

at the journal-bearing portion (as indicated in Fig 5.5) The hole at thecenter runs along the entire length of the camshaft and supplies lubricatingoil to the journal bearings

5.3.1 Chilled cast iron

The camshaft should combine a strong shaft with hard cam lobes Table 5.2lists five types of camshaft.4 Table 5.3 lists the chemical compositions of the

20 mm

various materials used The most widely used material for camshafts atpresent is chilled cast iron ((1) in Table 5.2), using a high-Cr cast iron Thistype of camshaft is shown in Fig 5.4, and has hard cam lobes with a strongbut soft shaft.

The chilled camshaft utilizes the unique solidification characteristics ofcast iron Figure 5.10 illustrates the production process Let us consider a

Material for rubbing surface

Adaptability

Young’s modulus

Hardness Strength

Unbalanced load caused by fluctuation of loading position

Contact area (surface treatment)

Spring force Vibration force

Cam profile

Valve train mass

5.8 Reasons causing pitting A tappet is the counterpart of the cam lobe in an overhead valve engine.

5.9 Camshaft cross-section at the position of an oil hole which is perpendicular to the central hole The holes supply oil to the journal bearing.

30 mm

T

Table 5.3 Compositions of camshaft matrials (%) The high-Cr cast iron is used for chilled camshafts The chromium concentration is slightly raised to obtain hard chill The hardenable cast iron generates a martensitic microstructure through quench-tempering The Cr-Mo steel SCM 420 is forged and carburized The sintered metal has a martensitic microstructure dispersing Cr and Fe complex carbide

High-Cr cast iron 3.2 2.0 0.8 0.8 0.2 – – – Balance Hardenable 3.2 2.0 0.8 1.2 0.6 – – – Balance cast iron

Cr-Mo steel 0.2 0.3 0.8 1.0 0.2 – – – Balance JIS-SCM420

Sintered metal 0.9 0.2 0.4 4.5 5.0 3.0 2.0 6.0 Balance for cam lobe

5.10 Casting process First, the electric furnace melts steel scraps, carbon content raiser (carbon powder) and ferro-alloys (Fe-Si, Fe-Cr alloys, etc.) Then the melt taken in the ladle is poured into the mold The mold is a sand mold A chiller is inserted in advance at the cam lobe position where chilled microstructure is required After

solidification, the sand mold is broken and the camshaft is taken out The sand mold contains a binder and appropriate water content It should be breakable after solidification without break during pouring The sand is reused The iron shots blast the shaped material to remove the sand The unnecessary gate, sprue and runner are cut The remnants are remelted and reused Grinding deburrs the shaped material Then it is directed to the final machining Casting is an extremely rational production process.

Sand mold

Molding

Electric furnace melting

Inoculation

Ladle

Cam lobe

Chiller

Science and technology of materials in automotive engines120

gradual increase in carbon concentration towards 4.3% in the iron-carbonphase diagram (Please refer to Appendices C and D for more detail on thephase diagram of the iron-carbon system.) Pure iron solidifies at 1,536 °C

The solidification temperature decreases with increasing carbon concentration

to give a minimum value of 1154 °C at a carbon concentration of 4.3%, the

eutectic point, (see Fig C.1)

Molten iron is transferred from furnace to molds using a ladle coveredwith a heat-insulating lining In manual pouring, one ladle of molten iron can

be poured into several molds one after another, which takes around fiveminutes If the solidification temperature of the metal is high, then the pouringmust be finished within a very short period of time otherwise the molten ironwill solidify in the ladle Hence, with a lower solidification temperaturethere is more time for pouring

Sand molds produce a slow solidification rate because the insulating effect

of the sand slows cooling Under these conditions, the carbon in the cast ironcrystallizes as flaky graphite (Fig 5.11(a)) and the casting expands Thisexpansion ensures that the casting fits the mold shape very well The resultantmicrostructure of the iron matrix becomes pearlite The microstructure offlaky graphite cast iron has sufficient strength for the shaft portion

By contrast, the cam lobe needs high hardness to provide good wearresistance If the rate of solidification of cast iron is fast, the included carbonforms into hard cementite (Fe3 C) Iron combines with carbon to form cementitebecause graphite is difficult to nucleate at high solidification rates Detailedexplanations are given in Appendix C Figure 5.11(b) shows the microstructureassociated with rapid solidification This microstructure is referred to asLedeburite or chill, it is very hard and is highly suitable for the hardnessrequirements of cam lobes

The cam lobe portion should be cooled rapidly in order to generate hardchill An iron lump called a chiller is used for this purpose The chiller ispositioned at the cam lobe and takes heat away from the casting, giving arapid solidification rate The chiller is normally made of cast iron Figure5.10 illustrates the relative positioning of the chiller and cam The chiller has

a cam lobe-shaped cavity and is inserted into the sand mold prior to casting.Except for the chiller, the master mold consists of compacted sand Theshape and volume of the chiller determine how effective it is at absorbingheat, and it must be designed carefully to give the optimum cooling rate.Figure 5.12 shows a section of a cam lobe, produced by etching thepolished surface with dilute nitric acid The pillar-like crystals, known as acolumnar structure, align radially at the periphery, whilst they are not seen atthe center In Fig 5.11(b), the columnar structure is aligned vertically, indicatingthat solidification advanced along the direction of heat flow The crystalformation process during solidification was discussed more fully in Chapter2

Figure 5.13 shows the distribution of hardness at a cam lobe section,measured in three directions from the center to the periphery The convexportion of the cam lobe shows a hardness of around 45 HRC, which is sufficientfor this application, whilst the central portion is softer at 25 HRC Thesemicrostructures correspond to the chill of Fig 5.11(b) and the flaky graphite

5.11 (a) Flaky graphite of a shaft portion and (b) the chilled

microstructure of a cam lobe Chill has a mixed microstructure of cementite (white portion in (b)) and pearlite (gray portion) The hardness is around 50 HRC Austenite and cementite appear

simultaneously by eutectic solidification The austenite portion transforms to pearlite during cooling The eutectic solidification is called Ledeburite eutectic reaction The additional quench-tempering changes pearlite to martensite This heat treatment raises the

hardness to 63 HRC.

(a)

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of Fig 5.11(a), respectively Generally, solidification starts from the surface,where the cooling speed is faster Solidification in the central portion is slowdue to the slow heat discharge rate, as confirmed by the hardness distribution

5.12 Macrostructure of a cam lobe The hardness measurement has indented small dents.

by the chiller The chiller has contacted the molten cast iron only at the gray part in the illustration.

Chill

Flaky graphite

10 mm

It is not easy to produce hard chill without any graphite in mass production.

A chill microstructure including graphite is soft and defective Themanufacturing process must control the chill hardness of the cam lobe toachieve the required value (45 HRC), while avoiding hard chill in the shaftportion Hard chill in the shaft portion can break an expensive gun drill insubsequent machining operations, as described below Generally, insufficientconcentrations of C and Si are likely to cause chill even at slow solidificationrates

Inoculation5 is a procedure aimed at solving the paradox that the hardchill and soft but strong shaft go together As shown in Fig 5.10, the inoculant

is placed in the ladle before pouring The inoculant adjusts the graphiteshape (see Appendix D), preventing chill where it is not required The inoculant

is an alloy powder, such as Fe-Si, Ca-Si, or an alloy containing rare earthmetals The inoculation effect lasts for a limited period of time and graduallydisappears after the inoculation (known as fading), thus it is important totime the inoculation accurately The effect is similar to that of a nodularizer,

as discussed in Chapter 2

An alternative process to ensure hard chill at the cam lobes is remelting(see (2), Table 5.2) This process controls casting and chilling separately.The material is first cast to produce the flaky graphite microstructure Thenthe surface of the cam lobe is partially remelted and then solidified rapidly

to generate chill Although it requires an additional process, this methodprovides better control of the hardness However, if remelting is too slow, itcan cause the shaft section to melt, so a high-energy heat source, such as atungsten inert gas (TIG) torch, is used The concentrated heat melts thesurface of the cam lobe instantaneously

5.3.2 Analysis of chemical composition of cast iron

before pouring

Reusable raw materials, such as steel scraps from the body press process, are

an abundant by-product of car manufacturing (Fig 5.10) An electric furnace6melts the scrap with carbon (to raise carbon concentration) and ferro-alloys.The chemical composition must be checked before pouring If molten castiron has a high oxygen content, this will lower the strength of castings.Carbon and silicon7 are used to remove oxygen from the melt, by reactingwith oxygen to form CO2 (which comes off as gas) or SiO2 (which forms aglassy slag) In addition to this deoxidation effect, both elements greatlyinfluence the strength of products through changing graphite shape anddistribution

Carbon concentration decreases rapidly in the melt, whereas siliconconcentration does not The concentration of other elements present, such as

Mn, Cu, Ni, etc., does not change This means that the carbon concentration

Science and technology of materials in automotive engines124

in the melt must be analyzed and adjusted as necessary just before pouring.Analysis of the carbon concentration in the melt is based on a carbon equivalent(CE) value, obtained by measuring the solidification temperature of the castiron When molten cast iron is cooled, the gradient of the cooling curvebecomes zero at the temperature at which solidification starts, due to thelatent heat of solidification A schematic example is shown in Fig 5.14.Cooling continues when solidification is complete The cooling curve ofwater shows similar behavior at the freezing point (0 °C), where ice and

water coexist If the water contains salt, the freezing temperature is lower,and solidification begins at a lower temperature The same can be observed

in the solidification behavior of cast iron

Stagnation caused by latent heat

T

Time 5.14 Cooling curve indicating temperature vs time A stagnation appears at the solidification point.

The solidification temperature of cast iron is proportional to the sum ofthe percentage of carbon and one-third of the percentage of silicon This isknown as the CE value and is measured using a CE meter If the CE value islower than that expected, the cast iron has a low concentration of carbon andsilicon A camshaft should chill only at the cam lobe, but if carbon andsilicon concentrations are too low,8 chill will be generated at the shaft portion

as well Measuring CE helps to reduce the risk of subsequent failure